This invention relates generally to telephone test circuits and methods. In particular, it relates to a test circuit or device with which to test the longitudinal balance of a telephone line. The invention al,so relates to a method of testing a telephone line for longitudinal balance, wherein the testing accommodates changes in a test signal that occur when the test signal is applied to telephone lines having different lengths.
A telephone line with which the present invention can be used includes two wires, each of which is typically insulated and twisted with the other to define a twisted wire pair. Typically, several pairs are grouped in a cable that has a surrounding outer conductive shield that is connected to earth ground when put in use.
Such a cable can be used, for example, between a central office and a crossconnect box in a telephone system. Another such cable can extend from the crossconnect box into the user territory where individual pairs are routed to respective end users. In this example, one end of a particular pair connects to a residential crossconnect box and the other end connects to a conventional telephone set in a house. A corresponding pair in the other cable connects to the crossconnect box at one end and to the central office at the other end. Cross connections at the box connect the pair running to the house with the pair running to the central office.
Each such pair of wires preferably has certain characteristics that facilitate clear transmissions of voice or data signals. These are well known in the industry, and there are various test devices to check the characteristics. One of these characteristics is referred to as longitudinal balance.
Longitudinal balance relates to the difference in voltage between the two wires of a pair (i.e., the responsive xe2x80x9cmetallic voltagexe2x80x9d) that arises in response to a voltage that might arise between the wires and earth ground (i.e., the disturbing xe2x80x9clongitudinal voltagexe2x80x9d). An example of a longitudinal voltage source in the environment of the telephone system is the alternating current transmitted through a public utility""s power transmission system that runs close to telephone lines of the telephone system.
The Institute of Electrical and Electronics Engineers, Inc. has published a standard for measuring longitudinal balance. It defines the degree of longitudinal balance as the ratio of the disturbing longitudinal voltage Vs and the resulting metallic voltage Vm of the network under test expressed in decibels, namely:
longitudinal balance=20 log10|Vs/Vm| dB,
where the voltages are of the same frequency.
An equivalent way of determining longitudinal balance is to have test equipment generate the disturbing longitudinal voltage Vs and use the responsive metallic voltage stated in decibels, specifically dBrnC (i.e., decibels above reference noise, as measured with C-message weighting) which is referred to as xe2x80x9cstressed noise.xe2x80x9d If this value is below a respective predetermined value, then there is a minimal level of noise arising from the disturbing longitudinal voltage and thus there is acceptable longitudinal balance. It is this approach that has been used in a test device, model no. 8336H, manufactured and sold by communications Manufacturing Company of Los Angeles, Calif. under the trademark xe2x80x9cPAIRCHEK.xe2x80x9d This device displays the xe2x80x9cstressed noisexe2x80x9d value. If the value is equal to or less than the respective predetermined value, the telephone technician testing the line knows the line is acceptably balanced (i.e., if the stressed noise is below the maximum acceptable limit, then the longitudinal balance is above the minimum acceptable limit).
In such a known type of test device, there is a circuit that generates an alternating current test signal as the disturbing or driving longitudinal voltage that is applied to the telephone line under test. This voltage varies with differences in the lengths of the telephone lines tested. That is, for a given test unit that is to generate the same driving longitudinal voltage from test to test, the driving longitudinal voltage in fact changes in response to the length of the telephone line to which the unit is connected for testing. This in turn means that the maximum acceptable level of stressed noise must change if a constant minimum longitudinal balance level is to be maintained.
The present invention provides a novel and improved longitudinal balance test circuit and test method that enable a telephone technician to use a single test device to accurately test for longitudinal balance in telephone lines of different lengths despite the driving longitudinal voltage output by the test circuit changing due to different lengths of telephone lines tested.
In another aspect of the present invention, a novel and improved test circuit that can be used to perform the method of the present invention (or other methods for which it is suited) includes a coupling transformer to provide the generated driving voltage to the pair of wires of the telephone line under test. This longitudinal balance test circuit of the present invention comprises a transformer including a first winding that connects to a telephone line. This transformer also includes a second winding that connects to a voltage source providing an alternating current test voltage with which to test longitudinal balance of the telephone line. In a particular implementation, the first winding is adapted to connect at one end to tip and ring lines of the telephone line (specifically through matched impedances) and to connect at another end to a shield conductor of the telephone line.
Stated another way, the present invention provides a device to test a shielded telephone wire pair. The device comprises a first connector to connect to a first wire of the shielded telephone wire pair, a second connector to connect to a second wire of the shielded telephone wire pair, and a third connector to connect to the shield of the shielded telephone wire pair. The device further comprises an alternating current test signal generator and a transformer. The transformer includes: a first winding having one end connected to the first and second connectors and having another end connected to the third connector; and a second winding connected to the signal generator. The device still further includes an indicator connected to the first and second connectors.
The method, of testing a telephone line for longitudinal balance in accordance with the present invention comprises: measuring an alternating current test signal applied to a telephone line under test; measuring stressed noise of the telephone line in response to the applied alternating current test signal; calculating, in response to the measured alternating current test signal and the measured stressed noise, an adjusted value related to longitudinal balance regardless of the length of the telephone line under test; and displaying the adjusted value.
Another statement of the present invention is as a method of testing a telephone line for longitudinal balance comprising: connecting a test unit to the tip line, the ring line, and a shield of a selected telephone line to be tested; generating in the test unit an alternating current test signal and applying the test signal to the selected telephone line connected to the test unit; measuring in the test unit the alternating current test signal applied to the telephone line; measuring in the test unit stressed noise of the telephone line in response to the applied alternating current test signal; and calculating in the test unit, in response to the measured alternating current test signal and the measured stressed noise, an adjusted value to indicate whether the telephone line is or is not in balance regardless of the length of the telephone line under test.
Although not limiting broader aspects of the present invention, a particular aspect of the aforementioned calculating step is solving in a computer the equation (xxe2x88x92(txe2x88x92n))+y, where x=a predetermined minimum acceptable longitudinal balance value (in dB), t=the measured value of the longitudinal test signal (in dBrnC), n=the measured value of stressed noise (in dBrnC), and y=a predetermined value representing a maximum acceptable stressed noise level (in dBrnC).
Therefore, from the foregoing, it is a general object of the present invention to provide a novel and improved longitudinal balance test circuit and method of testing a telephone line for longitudinal balance. Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art when the following description of the preferred embodiments is read in conjunction with the accompanying drawings.